Planetary rolling mills of the type described above have been previously known. However, these known types of rolling mills do not provide for effecting control of the flatness of the strip to be rolled. This is particularly important because the there is an increasing demand for strips with a high degree of flatness and a smaller thickness tolerance over the length of the hot rolled sheet.
Flatness defects which are evident after the rolling process, as is known, are essentially due to different stretching conditions over the width of the strip due to nonuniform shaping of the strip in the roll nip. These defects are noticeable as local undulations in the strip, occurring either at the edges of the strip or in the center thereof.
In conventional roll stands, control of the flatness is effected in various ways, primarily by mechanical or thermal measures. Known proposals contemplate, for instance, a barreling of the rolls directed in the directed opposite the sag of the rolls under load, while other solutions operate with a pair of rolls in which the rolls swing against each other or with bending means which act on the rolls.
Insufficient flatness of the strip results if a change in the profile shape takes place upon rolling a starting cross section to a final cross section. This can be remedied by applying tension to the strip, whereby the strip is subject to plastic deformation forces, as a result of which the undulations produced by deficiencies in flatness disappear. Such plastic deformation, however, also changes the thickness of the strip and thus leads to a poorer thickness tolerance. In other words, thickness variations may be reduced by stretching the strip, but this also results in a narrowing of the strip.